What is the difference between impressions and reach?
Impressions count how many times content was displayed. Reach counts how many distinct people saw it. If one person sees an advertisement five times, that is five impressions and a reach of one.
The relationship between them gives you frequency, calculated as impressions divided by reach, which is the average number of times each person saw it. Frequency is often the more actionable number of the three, because it tells you whether you are reaching new people or repeatedly showing the same audience.
High frequency is not inherently wrong. Some recognition benefit comes from repetition, and a few exposures generally outperform one. Beyond a point it becomes wasteful and can actively irritate, an effect known as creative fatigue, where click-through declines as the same people see the same message repeatedly. Watching frequency rise while performance falls is a clear signal to refresh the creative or widen the audience.
A practical caution: reach is harder to measure accurately than it appears. A person using a phone and a laptop may count as two, and platforms differ in how well they resolve this. Reach figures are therefore best treated as approximations and compared within a platform rather than across them.
The distinction also affects budget decisions. If reach has plateaued while impressions keep climbing, additional spend is buying more exposures to an audience already saturated rather than finding new people. That is the moment to change targeting rather than increase budget.
Neither metric says anything about outcome. Both are measures of exposure, and a campaign can accumulate impressive numbers of each while producing nothing of value.